fix(calendar): string selectors for start/endCalendar did not work

In case the startCalendar/endCalendar settings were given as string (which basically was supported) the current code assumes the selector would have the same parent as the opposite calendar, which is not even true in the docs-example (where each calendar is inside a form div)
This PR now corrects this, so it is using the given string selector as expected searching in the whole document

Closes #685

src/definitions/modules/calendar.js

@@ -646,7 +646,7 @@ $.fn.calendar = function(parameters) {
               return null;
             }
             if (!(selector instanceof $)) {
-              selector = $module.parent().children(selector).first();
+              selector = $(selector).first();
             }
             //assume range related calendars are using the same namespace
             return selector.data(moduleNamespace);